1. Field of the Invention
The present invention concerns a neutron backscatter instrument for detecting and identifying the contents of a vessel. The instrument is especially adapted for use in water, i.e. for sub-sea measurements but may be useful in other environments.
2. Description of the Related Art
It is frequently a requirement to identify and estimate the level of fluids (including vapour and liquids) such as water or hydrocarbon fuels in vessels located underwater. For example, it may be necessary to measure the amount of water in a buoyancy tank. Another important application for sub-sea measurement is in the salvage of submerged or sunken sea-going marine vessels or wrecks, in order to locate fuels for recovery and to avoid leakage into the environment as the vessel structure degrades over time. It may be required to recover fuel, for example, as part of an environmental protection scheme or maritime emergency response. Neutron backscatter measurement involves emitting fast neutrons from a neutron generator into a material, then detecting and measuring the number and/or energy of the thermal neutrons scattered from the material as a result of interaction of the molecules of the material with the fast neutrons in order to provide information about an object, including the contents of a vessel. One problem with using neutron backscatter measurements in a hydrogen-rich environment, for example underwater, is that most neutron detectors are very sensitive to thermal neutrons scattered from hydrogen-rich materials such as water. Therefore the sensitivity of the detector to other materials is reduced because of the large number of counts received from the water. It is an object of the invention to overcome at least some of the problems associated with using neutron backscatter instruments in an underwater environment.
U.S. Pat. No. 3,716,711 describes a water-level gauge, using a neutron source, for indicating the water-level in a sunken vessel. The device is designed to be placed against the flat external wall of a vessel so that neutrons emitted from the source can enter the vessel and interact with its contents. Slow neutrons which are scattered by water present within the vessel are then detected by the detector within the water-level gauge. The gauge incorporates a moulded urethane shroud to provide buoyancy.